1. Field of the Invention
The present invention relates to a process for the surface treatment of moldings based on polyamides by treatment with aqueous acids.
The present invention further relates to moldings obtainable by this process and to the use thereof for metallization, i.e. for coating with metals.
Polyamides are engineering plastics and have found wide utility in many areas.
However, an interesting area has hitherto been largely denied to polyamides--the field of metallized materials. The reason for this is mainly the inadequate adhesion of the metal layer on the polyamide. This matter is discussed in a paper by Seldenspinner (Galvanotechnik 75 (1984), 852), and various possible solutions are tentatively hinted at, for example treatment with aqueous hydrochloric acid. In Galvanotechnik 79 (1988), 54, a process is described for the surface treatment of polyamide injection moldings wherein activation in a bath containing palladium salts is followed by surface roughening in a bath containing calcium and aluminum salts. This process is commercially disadvantageous because of the expensive materials used, nor is the adhesion obtained fully satisfactory.
2. Review of Related Art
EP-A 56 986 discloses polyamide molding compositions which contain from 30 to 60% by weight of wollastonite as filler and are suitable in particular for metallization. The pretreatment is carried out with a from 4 to 15% strength solution of hydrochloric acid or sulfuric acid, which by attacking the filler particles produces a roughening of the surface; that is, the filler is absolutely necessary for obtaining good adhesion between metal and polyamide. However, the high filler content impairs the quality of the surface, which is a disadvantage for decorative purposes.
EP-A 402 750 discloses a process of surface treatment and subsequent metallization for polyamides. In this process partly aromatic copolyamides are treated with aqueous H.sub.3 PO.sub.4 and metallized. However, partly crystalline or amorphous polyamides which can be composed of aromatic monomer units cannot be satisfactorily metallized by this process. The adhesion obtained is not sufficient for some applications.